After getting some advice from this forum and talking to some folks in Hood River, I decided to take a crack at fixing a big soft spot on my Mistral Flow deck. Take a look at what I did and place your bets on how long this repair will last.
1. It will fail the first time out.
2. It will last a while, maybe 5-10 sessions.
3. It looks ok. . . may last for years.

Here is the documentary. . .
I bought the board used for $150 last summer. I didn't notice a big soft spot on the deck until I took the board out on the water several weeks later. The soft spot is between the front and back straps and the deck depresses about 0.25 - 0.75 inches.

Here is a picture of the board with the soft spot outlined in pencil. Wow! That's a big soft spot.

I taped off the area and drilled a bunch of holes through the glass. Then I filled the holes using an expanding polyurethane foam I got from West Marine. It filled very well, and I could see when I had added enough because it would start coming out adjacent holes. At first I was concerned that it wouldn't have enough structural strength, but it actually hardens up pretty good and has some resilience too.

Pic after filling with foam.

Next I removed the masking tape, sanded down to the fiberglass in a small area around each hole, and filled with West System G-Flex epoxy.

To finish it off I tried to find approximately matching paints, painted it, and added a little fresh non-skid coat.

I know it's not the most professional way to do this kind of repair, and my paint job is pretty bad, but I am not hung up on the looks of the board. Overall, I am pleased with how it turned out structually. The board feels strong. There is no noticeable difference between the deck flex/stiffness in the repaired area vs. the normal area. We'll see what happens when it is exposed to the dynamic loads of sailing. I am 200lb, and I sail in the Gorge with modest jumping involved. So this board will get some stress.

Place your bets now on how long it will last. For extra credit, predict the failure mode.
1. Foam and deck will delaminate.
2. Deck will develop new soft spot in same area.
3. Epoxy fills will fail and let water in or start cracks in the deck.

The step that you left out that I would have done would have been the addition of a fiberglass lamination over the holes. That unify everything and essentially eliminate cracks or any localized failure at the holes. However, I've never used the West System G-Flex epoxy, and maybe it would tend to resist cracking that could be more possible with their 105 epoxy.

Overall though, it's unclear how long your fix will last. It is important to remember that the cause of the delamination is a breakdown of the internal EPS foam. Although you have incorporated some foam stringers and localized fill, the compromised EPS is still there, and it may continue to flex and soften over time. I've had great luck with a couple epoxy injections that Mike Zajicek did on a couple my Mike Lab boards just to get some more time out of them. One of them is still going strong 7-8 years after the injection. It's normally not an optimum fix, but it can work in some instances.

Agreed: you really really should have laminated a layer of s-glass over the entire area with holes. Epoxy is strong, but brittle. In a composite, the mesh (fiber, metal, glass, plastic, etc...) carries the load, and the resin (epoxy, polyester, acrylic, glass, etc...) acts to hold everything in the desired geometry. A couple of good landings and you may have some cracks and soft spots again.
The other thing, is that expanding foam is surprisingly porous within the spaces it's injected. The gasses evolved to cause it to expand often have no way to escape and create fairly large voids. This is great for insulation and keeping bugs/water/air out of cracks in structures, but terrible for fixing a soft spot like this. I have fixed similar delamination/soft-spots by injecting epoxy, rather than expanding foam. You will probably still have a dent there, but it'll be a hard dent. I'd be interested in hearing how this repair holds up; please report back!_________________Kansas City

Yes, I would have glassed over the whole repair then covered your work with a matching piece of white pad material from NSI. It would be less work than matching the paint, helps cushion the area from future stresses, and is quite comfortable (and grippy) when sailing/jibing with your back foot out of the straps.

The soft spot between the straps is such a common problem on older boards, I don't know why all board makers don't pad the area between the straps to begin with.

I'm voting for 10 sessions if you jump the thing anywhere above 6 ft
and the failure mode will be Foam and Deck will delaminate (sorta, actually
the underlying polystyrene board blank will compress more, and
"delaminate" from the expanding foam you put in, and the spot will get soft
again.

Hope it lasts for years.

-Craig

Jim.od3 wrote:

1. It will fail the first time out.
2. It will last a while, maybe 5-10 sessions.
3. It looks ok. . . may last for years.

Place your bets now on how long it will last. For extra credit, predict the failure mode.
1. Foam and deck will delaminate.
2. Deck will develop new soft spot in same area.
3. Epoxy fills will fail and let water in or start cracks in the deck.

Yes, I would have glassed over the whole repair then covered your work with a matching piece of white pad material from NSI. It would be less work than matching the paint, helps cushion the area from future stresses, and is quite comfortable (and grippy) when sailing/jibing with your back foot out of the straps.

The soft spot between the straps is such a common problem on older boards, I don't know why all board makers don't pad the area between the straps to begin with.

I would have used this approach. I think the problem with a production board like this is that with all those inserts, the delam will probably move over to them and cause leaks around the holes that will suck water inside, then the board dies even faster. Using liquid foam is unpredictable as to how much it will actually fill since you can't see it.

I'll chime in here. My firend did a similar fix on a deck of a sailboat. (Two layers of glass top&botto with a 1/2 inch foam core). His foam core was completely gone disenergrated over time. He drillholes and used the spray foam. This made the deck rigid again, where before with no foam core was flexing like a trampoline. He repair is overnten years old. I know his is a deck of a boat....but his repair was over 3 square feet of decking with no supporting structure underneath. We stand on it jump on it....no cracking or failure of any kind.

I think your repair will hold up better than you think. And if it does start to crack i time...seems to me that you are ahead of the game with ionly being 150 dollar board. So my bet....will last a long time !!!!

I'd wager that the rear footstrap insert is compromised given your indication of the soft spot and that there is no other noticable damage. Having said that I bet the area under the rear pad is waterlogged too. I would be concerned about the stringer being comprimised and saturated with water. My bet is 15 sessions or less if you are doing bump and jump. Keep us posted (and emergency flares strapped to your back).

I seriously considered adding a layer of reinforcing fiberglass over the repair area. I'm sure that would have been a better approach. If I have problems with this repair I'll probably strip it down and start over one more time . . . if just for the learning experience. I've never really worked with fiberglass, epoxy, foam before, so this was a nice little project for me.

That's why, when I find a great board, I hoard sound backups. Flows, like my favorite board, have weak decks. My whole floor-to-ceiling stack of the latter cost me less than a new board, I'll get some of it back when I sell them, I don't have to bother repairing them, and when I break one I just remove its straps and custom pads and stick 'em on the backup.

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